Class UnknownFieldSet.Field

A Field consists of five lists of values. The lists correspond
to the five "wire types" used in the protocol buffer binary format.
The wire type of each field can be determined from the encoded form alone,
without knowing the field's declared type. So, we are able to parse
unknown values at least this far and separate them. Normally, only one
of the five lists will contain any values, since it is impossible to
define a valid message type that declares two different types for the
same field number. However, the code is designed to allow for the case
where the same unknown field number is encountered using multiple different
wire types.